In regard to driving rotary members, more especially rotary members which are of large mass and/or which have a large periphery, being driven by an electric motor by way of a belt transmission, a problem which often arises is that of bringing the motor up to a reference speed of rotation within a reasonable time, without an excessive amount of slip at the belt. In that connection, there is the danger that, if the acceleration applied is excessively high, the drive belt will run hot as a result of slip and will accordingly suffer from a high rate of wear. That problem is even worse in regard to decelerating the rotary member in order to bring it to a stop. If the motor is simply switched off, the rotary member then continues to rotate by virtue of its inertia, due to its substantial mass, and it may be up to half an hour before the rotary member comes to a stop, when the rotary member is of considerable mass. If the motor is switched off when the rotary member is stationary, the motor reverses for a long time and begins to drive the rotary member in the reverse direction. In that respect, in both situations, a considerable amount of heat is generated, and not only when accelerating the rotary member to its reference speed of rotation but in particular also when braking the rotor, and a considerable amount of time is required to stop the rotor.